A British corporate investigator and his American wife, who disappeared into the Chinese judicial system a year ago, will be tried in a closed court next month, a family friend has told the Guardian.

The news is the latest development in an increasingly sensational affair, involving corruption, a sex tape and a televised confession, which has cast a shadow over the country's foreign business community since local authorities levelled explosive accusations against the British pharmaceuticals group GlaxoSmithKline (GSK) last summer.

The trial of Peter Humphrey and Chinese-born American Yu Yingzeng will be closed to family members and consular officials "on the grounds of privacy", said the friend, who requested anonymity because of the case's sensitivity.

The couple were detained last July soon after Chinese authorities accused GSK – one of Humphrey's clients – of bribing doctors and hospital administrators to sell its products. GSK had employed the couple three months earlier to investigate emails containing allegations of widespread bribery at the company – and a sex tape featuring the then head of GSK China.

"I am very worried that family and consular officials are not allowed to attend my parents' trial," the couple's 19-year-old son, Harvey Humphrey, said on Wednesday after US consular officials visited his mother. "This does not involve state secrets. This does not involve national security. It is about two private individuals, my parents.

"I am surprised at this decision since China wants to promote openness and the rule of law, and I hope that they will relent and let me in. I haven't seen them for a year. I am shocked and upset. I miss my parents, who are not in good health."

The trial date has been pushed back from 29 July to 7 August, Humphrey said.

Peter Humphrey, a former reporter, operated the Shanghai-based consultancy ChinaWhys with his wife from 2003, conducting fraud investigations and risk management services for international companies. According to the company's website, Humphrey spent two decades as a Reuters journalist in Asia, eastern Europe and the Balkans before becoming a "risk management specialist and corporate detective"; Yu had served as "a high-level advisory consultant in China".

They were detained on suspicion of operating an illicit business and illegally purchasing private information on Chinese citizens. Chinese authorities have not publicly announced any connection between the GSK investigation and their case.

The charge of operating an illicit business has been dropped, according to the friend.

In September, the couple were paraded on state television in handcuffs and orange prison vests. "The way we acquired information was sometimes illegal," Humphrey said in Mandarin. "I feel very regretful about it and want to apologise to the Chinese government." While televised confessions are common for disgraced Chinese officials, they are vanishingly rare for foreigners.

The London-based NGO Fair Trials International expressed concern over the closed hearing. "In China, whose supreme people's court proclaims an astonishing 99.5% conviction rate, criminal justice is used as a way of reinforcing state control, rather than finding truth and pursuing justice," said Jago Russell, the group's chief executive.

"Despite the UK's growing economic links with China, the UK government should be raising its concerns about China's clear violations of international fair trial standards – like parading Peter Humphrey on national television to 'confess' to his crime."

In March 2013, a video showing GSK's then-China boss Mark Reilly having sex with his Chinese girlfriend in his Shanghai flat was sent to 13 GSK executives, including its CEO Andrew Witty, the Sunday Times reported this week. It is still unclear who shot the video, and why. GSK considered the video a security breach, and Reilly hired Humphrey's firm to trace its origin.

GSK did not tell Humphrey about the bribery allegations until two months later, the newspaper reported.

Last month, Chinese authorities formally accused Reilly of running "a massive bribery network", implying that he could face jail time. He had left China last June after Chinese authorities raided GSK offices in Shanghai, but returned in October to aid in the investigation. Reilly has been barred from leaving the country, and his whereabouts are unknown.

GSK said in a statement this week that it is cooperating fully with the Chinese authorities' investigation into the bribery allegations, which is ongoing.